1. Field
This disclosure relates generally to radar image processing, including methods and apparatus for creating and processing universal radar waveforms.
2. Background
Radio detection and ranging (radar) technology transmit and receive pulsed electromagnetic waves to detect objects. In radar, an electromagnetic pulse is transmitted from a radar platform, and this transmitted pulse can be scattered back to the radar platform by various objects. The roundtrip time taken by the pulse to travel from the radar platform to the scattering object and back therefrom may be proportional to the distance between the radar platform and the object. The transmitted pulse may generally be sent with a characteristic waveform such that the returned pulse will be scattered back with a shape resembling this transmitted waveform. Multiplying the returned energy by this waveform may allow the returns to be sampled in time to yield a single complex number representation for the returned energy as a function of time—referred to as complex radar pulses. Since the complex radar pulses may be a function of time and the distance may be proportional to the roundtrip travel time of the pulses, these complex radar pulses can be thought of as samples of the objects present at various distances from the radar. This is how distance can be measured using a single radar pulse.
If two pulses are used, the line-of-sight motion of an object can also be determined. If the object is at a certain distance from the radar platform and is moving toward the radar platform, then the complex radar pulses may rotate in phase with a positive frequency. Similarly, if the object is moving away from the radar platform, then the complex radar pulses may rotate in phase with a negative frequency. This effect is referred to as the Doppler effect. If multiple pulses at a given distance are used in combination, multiple Doppler frequencies can be measured by decomposing the complex radar pulses into sine and cosine waves at these frequencies. The energies observed at these various frequencies is called the Doppler spectrum.
However, if the radar platform is moving, then returns that appear at various Doppler frequencies can be the result of multiple stationary objects located at various different positions. For example, assume that a radar pulse is transmitted in a direction perpendicular to the motion of the radar itself and define the vector pointing from the radar to a stationary object that scatters back pulse energy as the pointing vector. If the pointing vector to this object is perpendicular to the direction of the radar motion, then the complex radar pulses for this object may appear at zero Doppler frequency. If, however, the pointing vector is positively aligned with the radar motion vector, then the complex radar pulses for this object may appear at a positive Doppler frequency proportional to the apparent line-of-sight motion of the object relative to the radar platform.
This separation of stationary objects within a Doppler spectrum obtained from a moving radar platform may have many different applications. For example, it is the foundation of what is called synthetic aperture radar (SAR) imaging. While the problems discussed herein are not limited to SAR imaging applications, SAR images provide an illustrative example useful for describing deficiencies in the prior art. Those skilled in the art will recognize that the problems addressed herein apply more globally to radar returns and the processing thereof, and are not limited to their use in SAR images, which is only one example.
In SAR imaging, images are produced by post-processing a series of complex radar pulses from a moving radar platform. In this imaging method, the Doppler resolution of a radar image (which relates to the distance between objects perpendicular to the radar line-of-sight, or distance in the cross-range direction) may be inversely proportional to the length of the temporal aperture over which the Doppler decomposition is performed (this decomposition is also referred to as coherent integration). Increasing the time over which coherent integration is performed may provide finer cross-range resolution, which may be a desirable quantity for SAR imagery.
Multiple intelligence, surveillance and reconnaissance (ISR) missions may require different collection modes that may be mutually incompatible. For example, a common type of ISR mission for which SAR radar platforms may be used is moving target indication (MTI) missions. MTI missions may require many pulses per detection opportunity along with a narrow beam. In contrast, detailed imaging missions may require the collection of continuous pulse streams using broad beams. As these modes may be inherently incompatible, no MTI information may be available during image mode collection and vice versa.
One approach that may be taken to address the incompatible modes of operation under which the radar platform is required to collect data is to have the radar platform transmit pulses for both modes over the same time period. However, assuming that a fixed number of samples may be transmitted and collected over the same time period, the radar resources collected over the same time period for each mode may be effectively halved. This can result in a dramatic, corresponding decrease in the quality of resulting SAR images.
In short, generating high quality SAR images using existing radar imaging schemes may require numerous radar resources. For example, existing radar imaging schemes may be directed to using sampling patterns that allow for straightforward processing to generate the radar data output. While existing sampling patterns may have known and straightforward processing techniques, they may require a high number of pulses and a large amount of radar resources. Any reduction in the number of pulses may result in significant degradation in processed radar returns.
U.S. patent application Ser. No. 12/026,508, entitled “Method and Apparatus for Creating and Processing Universal Radar Waveforms,” filed Feb. 5, 2008, attorney docket number 81085-12, discloses a time domain approach that addresses these problems, the entire content of which is incorporated herein by reference. However, applicants have since discovered other approaches which provide even greater flexibility.